(1) Field of the Invention
This invention relates to an improved process for activating of steel and to an aqueous activating solution utilized for acceleration of the formation of phosphate coating as well as for refinement of phosphate coating crystals prior to phosphating, particularly zinc phosphating treatment which is applied to steel surfaces prior to the painting, particularly electropainting.
Such activating treatment by the use of an aqueous activating solution containing titanium ion and colloidal titanium (hereinafter reffered generically to as "titanium") has been effected for accelerating the formation of the phosphate coating on steel surfaces by means of activation of the surface thereof as well as for refining phosphate coating crystals in the phosphating treatment process for the surfaces of the steel and surface treated steels represented by zinc plated steels and the like are to be fabricated. An example of the process as mentioned above includes the following steps:
alkaline cleaning.fwdarw.multi-stage water rinsing.fwdarw.activating treatment.fwdarw.phosphating treatment.fwdarw.multistage water rinsing.fwdarw.deionizing water rinsing.fwdarw.drying-off.fwdarw.painting (cathodic electro painting, anodic electro painting, dip painting, spray painting and the like).
(2) Description of the Prior Art
Activating compositions utilized for activating steel surfaces have been publicly known in, for example, U.S. Pat. Nos. 2,874,081, 2,322,349, 2,310,239 and so on and which disclose titanium pyrophosphate ion, orthophosphate ion, sodium ion and the like contained in activating compositions as their main components. There were, however, the following disadvantages in the case when an aqueous activating solution prepared from conventional activating compositions is applied prior to phosphating treatment.
The first disadvantage is that while a comparatively dense and fine-size crystalline phosphate coating can be formed in a phosphating treatment by application of an aqueous activating solution immediately after fresh buildup, but a coarse crystalline phosphate coating or phosphate coating is obtained on the steel surface by the application of the aqueous activating solution of 2 or more day lapse after buildup (the lapse of 2 or more days involves either a case where such time elapses after the buildup with the use of the solution for activating treatment, or a case where the solution is allowed to stand for 2 more days without employing after the buildup). In other words, the effect of formation of the fine-size crystalline phosphate coating as mentioned above is easily reduced, so that there was a disadvantage in that such aqueous activating solution exhibits deterioration with time. For the sake of reviving to a aqueous activating solution in such performance being substantially the same as that of the initial solution immediately after buildup, even if an activating composition is replenished in the aqueous activating solution, such effect as described above cannot be attained. For this reason, such aqueous activating solution must frequently be discarded. This is obviously disadvantageous in view of control of the aqueous solution and economy.
The second disadvantage is that while the above-mentioned deterioration with time can be suppressed by increasing the concentration of pyrophosphate ion in such aqueous activating solution to 60 ppm or more, on the other hand, pyrophosphate ion reacts with the steel surface to form an inactive film, i.e., so-called inactivation of steel surface is observed. Thus, when the steel surface with the inactive film is subjected to phosphating treatment, an non-uniform coating of blue color, white spot and the like is easily formed so that an uniform phosphate coating is hardly formed. When such non-uniform coating is electro coated in, for example, the following stage, a surface roughening of the electro coated film is induced and it results in disadvantages such as deterioration of corrosion resistance, particularly filiform corrosion resistance of the paint film. The reaction of pyrophosphate ion with steel surface as mentioned above depends on the type of steel. For instance, since drawing quality bake-hardenable high tension steel has higher surface activity than that of JIS-G-3141 SPCC, the former reacts easily so that the former steel has a tendency to easily form such a non-uniform coating as described above on the surface thereof.
The disadvantages as mentioned above cannot be overcome by adding an alkaline phosphate such as disodium phosphate, trisodium phosphate or the like to the aqueous activating solution, or by increasing the titanium concentration in the aqueous solution. Thus the aqueous activating solution prepared from conventional activating compositions involves said disadvantages which are difficult to overcome technically.